CPAP apparatus is well recognized for use in the treatment of a number of respiratory conditions such as, for example, obstructive sleep apnea and hypopnea. The apparatus applies a continuous positive pressure through a mask or a nasal cannula to a patient's respiratory system while the patient sleeps. The positive pressure acts as a pneumatic splint for expanding and preventing blockage of the upper airway. Typical CPAP apparatus includes a blower which produces pressurized air, a mask or nasal cannula and a hose connecting between the blower and the mask or cannula. The apparatus also includes a pressure controller. The CPAP pressure is measured either at the mask or at a base unit as delivered to the hose. The pressure is compared with a stored prescribed pressure and errors are used to adjust the pressure, typically by controlling the speed of the blower. The pressure controller also may be programmed to provide variations to the applied pressure, generally either based on time or based on patient need. The applied pressure has been controlled to provide an initial low positive pressure to make the patient more comfortable while falling asleep. As the patient falls asleep, the pressure is ramped up to the prescribed pressure either over a set period of time or after a set low pressure delay time. It also is known that the patient's breathing may be monitored and that the pressure may be automatically adjusted to increase the applied pressure in response to the detection of apnea and/or precursors to apnea, such as snoring, and to gradually decrease the applied pressure in response to the absence of apnea and/or snoring. Apparatus of this type automatically adjusts to the lowest pressure necessary to maintain airway patency.
More advanced CPAP systems provide two air pressure levels to the patient's respiratory system, namely, an inspiratory positive airway pressure (IPAP) during inhalation and a lower expiratory positive airway pressure (EPAP) during exhalation. For most patients requiring CPAP therapy, a higher IPAP pressure is required to maintain airway patency during inhalation, and a much lower EPAP pressure is sufficient to maintain airway patency during exhalation. Often, the EPAP pressure may be at or only slightly above ambient pressure, while the IPAP pressure is generally set to a pressure greater than the EPAP setting to provide the therapy needed during inspiration. By providing bilevel operation with the lowest necessary EPAP pressure, the work required for the patient to exhale is reduced and therefore the patient's comfort is increased. This in turn promotes patient compliance with the prescribed therapy.
Bilevel CPAP systems typically use one of two methods for controlling pressure. In both systems, a breathing signal is established to determine when the patient inhales and exhales. While the patient inhales, the applied pressure is set to the prescribed IPAP level and, when the patient exhales, the applied pressure is set to the prescribed EPAP level. When a person breathes, there is a slight pause between inspiration and expiration. In some systems, the EPAP and IPAP levels are changed in response to the beginnings of inspiration and expiration and in others the EPAP and IPAP levels are changes in response to the beginnings of the pauses. Some systems modulate the speed of the blower to increase and decrease the applied pressure. In other systems, the blower is set to provide a pressure of at least as high as the higher IPAP level and a vent valve is modulated to reduce the pressure to the prescribed levels during inspiration and expiration. When the blower speed is modulated, the pressure quickly ramps up to the prescribed IPAP pressure when inhalation begins and quickly ramps down to the prescribed EPAP pressure when exhalation begins. There is a slight ramping effect when the pressure is changed due to the inertia of the blower when changing blower speeds. When a vent valve is modulated, there may be a more abrupt change between the IPAP and EPAP levels, resulting in a square wave shape to the applied pressure. In both types of systems, the waveform will tend to have a slight pressure dip at the onset of inhalation followed by a sudden increase in pressure as the level increases towards the desired IPAP level. A short duration pressure spike typically is present during the onset of exhalation followed by a sudden decrease in pressure as the level decreases towards the desired EPAP level.
In the past, each commercially available bilevel CPAP system has had a particular profile to the applied pressure waveform based on the particular response of the blower and/or vent valve and the related control circuitry. The waveform has not been adjustable to enhance the comfort of the patient or to allow the physician to modify the therapy. The only waveform control available to the physician or therapist is to set the prescribed IPAP and EPAP pressures.